Matt,

           I can be heard very well, or so I'm told.  My nav/com is an
       SL-30 with a
           Bob Archer antenna in the tail.  It was tested by a ham prior to
           installation and checked out very well (don't remember the
   numbers).
           Coax is RG-400 and is one piece from the antenna all the way up
       to the
           radio.  The fuel gauges are the only thing that seem to be
       affected by
           the radio.

           Mark


           RG400 is 50 ohm and is for a quarter wave antennas.


   http://www.gore.com/en_xx/products/cables/coaxial/optimized_rg400_coaxial_cable.html

           Regular Nav/com antennas for metal airplanes are quarter wave.

           "Depending upon height above ground, the influence of
   surrounding
           objects
           and other factors, our quarter wave antenna with a near perfect
       ground
           exhibits a nominal input impedance of around 36 ohms."

           http://www.electronics-tutorials.com/antennas/antenna-basics.htm

           I think what you have Mark is a half wave antenna.
           "A half wave dipole antenna does not need a ground plane and is
           nominally 75 ohms."

           Despite the fact it is working well this implies some
           reflected energy on the outside of the coax shield.

           ------------------------------------------------------------



   http://support.radioshack.com/support_tutorials/audio_video/cable-coax.htm

           Cables carried by RadioShack
           93-ohm Cables: RG-62, RG-62U
           75-ohm Cables: RG-11, RG-11U, RG-59, RG-59U, RG-6, RG-6U, RG-6QS
           51-ohm Cables: RG-9, RG-9U
           50-ohm Cables: RG-58, RG-58U, RG-8, RG-8U, RG-8M, RG-8X--

           Paul Lamar ...No rotor no motor.

       Paul,

       Antennas and feed lines need to be matched with the output of the
       radio.  In this case, everything is designed to be 50ohm.  If
   anything
       in the system was the wrong nominal impedance (75 or 93), the
   radiated
       output would be very low (less than 10% of normal) and the reflected
       energy (SWR) would be very high (>3), and he wouldn't be able to get
       out.

       Most aircraft com antennas are modified 1/4 wave - 1/2 wave would be
       pretty big - each radiator would be something like 46" long.
    One of the
       antennas on this page is probably what Mark has:

       http://www.aircraftspruce.com/catalog/avpages/archer_antennas.php

       Even if an antenna's length leads to the wrong characteristic
   impedance,
       you can connect the antenna via an impedance matching circuit
   (inductor
       and/or capacitor).  Some antennas include this in the mounting base.

       Since Mark's radio performance is good (and his installation sounds
       normal compared to other aircraft), I'm back to thinking it's a
       vulnerability in the sensor circuit(s).  Being a fiberglass ship
    (no
       Faraday cage) with the antenna mounted inside the fuselage, and
   without
       a ground plane, the electronics in the cabin will be quite a bit
   more
       vulnerable to disruption from the transmitting comm.


       Regards,

       Matt-

       Those are very interesting antennas and I agree with what
       you said about VSWR. Mark did not say what kind of antenna
       he was using.  I assumed it was a half wave metal tape.
       Many composite aircraft use metal tape  antennas as mentioned in the
       Jan 2008 Sport Aviation article.

       A half wave would be about 38 inches long. The quarter wave antennas
       are 19 inches long.

       If there is a mismatch the coax shield becomes part of the antenna.
       Since it is in the same bundle with the fuel tank wiring the RF is
       probably
       being coupled into the fuel tank wiring. It also radiates to the out
       side of
       a composite aircraft probably enhancing the antenna pattern. I was
       an antenna technician (among my many careers) in my younger days
   about
       50 years ago before I got to be an engineer so I forgot a lot :)

       http://www.microwaves101.com/encyclopedia/vswr.cfm

       "Just remember to divide the larger impedance by the smaller
   impedance,
       because VSWR is always greater than 1. Hey, this calculation is
   so easy
       you can do it in your head!!!"

       "Let's look at the special case where you mix up 50 ohm parts into
       a 75 ohm system (or vice-versa). In either case, the resulting VSWR
       is 1.5:1. Yes, we did that without a calculator. While we're at it,
       the reflection coefficient is:"   "=(75-50)/(75+50)=0.2"


       BTW that is less than the 2:1 claimed by the AC Spruce antennas.

       One thing you might have to worry about is what the power amplifier
       does since it is probably designed for 50 ohms coax.

       Paul Lamar



   19"?  Is that at 125MHz (period of 8ns)?  Light will go 3*10^8 m/s.  1
   wavelength = 8ns * 3*10^8 m/s * 39.37in/m = 94.49in.  1/4 wave =
   23.622in.  Is 19in based on 2meter (144-148MHz)?

   http://www.csgnetwork.com/antennagpcalc.html

   Somehow I thought that much impedance mismatch would lead to a larger
   SWR.  Thanks..  An SWR of 1.5 is still getting close to only
   transmitting half power, so it's not good.


   Matt-

   Perhaps they have some sort of device to shorten it. According to the
   SA article most UHF antennas are 19 inches. I'll measure the ones on
   my 182 next time I am out at the hangar.
   --
   Paul Lamar ...No rotor no motor.


I suspect there's some loading circuitry in the antenna..  Just like
rubber duckies are usually 1/4 wave and clearly they aren't nearly 1/4
wave long.  Or, if it's an antenna with dielectric covering, maybe they
have to be tuned with a shorter length - the dielectric changes the
resonant frequency - making the wire look longer, I think..


Matt-

Mark what kind of antennas are you using? If tape was the coax
connected in the center. I assume they and the coax are bonded in
so there is little you can do about changing anything having to do
with that.

The other thing I am curious about is the new box that converts
the fuel sensor value to a variable frequency pulse. Please look inside
and tell me the numbers on the chips if they are readable of course.
Some manufacturers mask the numbers sometimes so it makes it
more difficult to copy the circuit.

The diodes Aubrey mentions may be embedded in the chip or chips what
ever the case may be. In that case the only thing I think you can
do is put them in a shielded box and move it away from the coax.